Conventionally, there are techniques of controlling plural power supply devices. According to one related technology, for example, a combination of one or more power supply units capable of supplying the amount of electrical power used by a device are extracted based on a table that indicates efficiency concerning the amount of electric power that each power supply unit can output, and the on/off of the plural power supply units is controlled. For a power supply device equipped with a power supply unit having plural power supply modules of an identical capacity, there is a technique of driving the power supply modules in a redundant state with respect to a load and turning off the power supply modules according to load conditions. There is a technology of calculating the number of plural power supply units to be operated, based on the total current value indicated by a digital signal and the rated current value of individual power supply units, and turning on/off the plural power supply units. There is a technology of supplying a same reference voltage to all power supply units connected in parallel from a power supply control unit and at the same time, adjusting the output voltage of all power supply units. For example, refer to Japanese Laid-Open Patent Document Nos. 2009-201244; H9-204240; H10-201090; and 2000-099166.
According to conventional technologies, however, it is difficult to suppress decreases in conversion efficiency while being prepared to respond quickly in case a problem occurs with any of the plural power supply devices. For example, when a power supply device among the plural power supply devices is turned off, decreases in the conversion efficiency of the power supply devices that are supplying power can be suppressed, however, when a problem occurs with a power supply device that is supplying power, it takes time to turn on a power supply device once turned off.